Polyurethane foams, formed by the reaction of a polyisocyanate with a polyhydroxyl-containing compound in the presence of a suitable catalyst, are widely accepted as padding materials for cushions in furniture, automobiles and the like. Such foams typically bum readily, and considerable effort has been devoted to reducing the combustibility of the foams. One technique by which this may be done is through the use of additives to the foam that retard its flammability or help to extinguish the burning foam should it ignite. Known flame retardant additives include 2,3-dibromo-1,4-butenediol; tris(2-chloroethyl)phosphate and triethylphosphate, for example. However, a disadvantage of using the phosphate-containing additives is that often relatively large quantities of the expensive materials must be used, higher than about 1%. Additionally, phosphorus and halogen-containing flame retardants create a plasticizing effect which causes the polyurethane foam to be reduced in hardness, lower in compressive strength and increased in density so that the foam is detrimentally affected. Ideally, the load bearing properties of the foams, such as ILD--indentation load deflection and CFD--compression force deflection, should not be adversely affected by the presence of the additive. These conventional flame retardants are also somewhat volatile and may evaporate out of the polyurethane foam over time, thus decreasing the available fire retardancy. Finally, there are indications that these materials may be corrosive to certain metals on which the foams are applied.
Considerable research has been conducted on combustion resistant additives for polyurethane foams. For example, melamine has been used alone or together with conventional phosphate flame retardants. It is further well known to use polyvinyl chloride resins and related materials in polyurethane compositions as flame retardants either alone or in conjunction with materials such as antimony trioxide and antimony pentoxide. Zinc oxide (ZnO) and antimony trioxide (Sb.sub.2 O.sub.3) have also been used together for this purpose. Random bromostyrene-containing copolymers have also been mentioned as improving fire retardancy when used in conjunction with the metal oxides mentioned above and others.
In low density, flexible and semiflexible polyether urethane foams containing chlorine-containing polymers such as polyvinyl chloride, zinc oxide and antimony oxide, part of the chlorine-containing polymer can be replaced with a chlorinated paraffin provided a proportionate reduction is made in the amount of zinc oxide employed, according to U.S. Pat. No. 3,876,571. Such foams must still retain some finely divided, solid chlorine-containing polymeric resin, such as polyvinyl chloride (PVC), antimony trioxide and zinc oxide.
U.S. Pat. No. 3,931,062 to Cobbledick teaches that when a minor amount of an oxide, hydroxide or basic salt of certain metals from Group 2a of the Periodic Table is introduced into a formulation for low density flexible and semiflexible hot-molded flame-retardant polyether-urethane foam compositions which also contain finely-divided solid chlorine-containing polymeric resins, zinc oxide (ZnO) and antimony trioxide (Sb.sub.2 O.sub.3), and optionally a chlorinated paraffin, the resulting foams can be prepared commercially with good reproducibility and good stream stability. It is noted, however, that chlorinated paraffins are never used in the Examples therein, and that PVC, ZnO and Sb.sub.2 O.sub.3 are still required components.
Discoloration occurring during commercial production of large buns or slabs of low density, flexible and semiflexible polyether urethane foams containing chlorine-containing polymers such as PVC, zinc oxide and antimony oxide is addressed in U.S. Pat. Nos. 3,884,849 and 3,978,011. This discoloration is reduced or prevented by using a small amount of a zinc salt of an organic monocarboxylic acid or an organic mono dithiocarbamic acid having from 1 to 25 carbon atoms in each acid moiety of the salt. It was noted as an aside that optionally part of the chlorine-containing polymer could be replaced with a chlorinated paraffin provided a proportionate reduction is made in the amount of the zinc oxide employed. None of the Examples in these patents employed a chlorinated paraffin to demonstrate the concept.
Nonetheless, flammability of polyurethane foams continues to be an important issue, and improvements in this area are always sought after. While additives are useful in this regard, as noted, many must be used in such large quantities that the resulting foam characteristics are degraded. It would be an improvement in the art if a foam additive could be discovered which not only imparts combustion resistance to the foam, and which not only does not degrade the foam physical characteristics, but which may actually improve those characteristics.